Action: Amend the soil with fresh plant material or crop remains
Biodiversity: One randomized, replicated experiment from Belgium found increased microbial biomass when crop remains and straw were added.
Compaction: One study from India found improved soil structure when straw was incorporated. One before-and-after trial from the UK found that incorporating straw residues by discing (reduced tillage) did not improve anaerobic soils (low oxygen levels) in compacted soils.
Erosion: Two randomized, replicated studies from Canada and India measured the effect of incorporating straw on erosion. One found straw addition reduced soil loss, and one found mixed effects depending on soil type.
Nutrient loss: Four replicated studies from Belgium, the UK and the USA (one also controlled, one also randomized, and two also controlled and randomized) reported higher soil nitrogen levels when compost or straw was applied, but mixed results when processed wastes were added. One also found reduced nitrate leaching when straw was incorporated. One replicated study from China and the Philippines found mixed results depending on site.
Soil organic carbon: Six studies from China, Denmark and India measured the effect of incorporating plant material into the soil. All (including one replicated, two randomized, replicated studies, one controlled, randomized, replicated studies and one controlled before-and-after site comparison) found higher carbon levels when plant material was added. One found higher carbon levels when straw was applied along with NPK fertilizers. One also found larger soil aggregates. One replicated study from China and the Philippines found mixed results depending on site.
Yield: One replicated trial from Denmark found higher barley yield when straw was incorporated. One trial from the Philippines found higher grain yields when crop remains were incorporated earlier in the season.
Soil types covered: clay, clay loam, fine loam, loam/sandy loam, loamy sand, sandy, sandy clay loam, sandy loam, sandy-silt, silt loam, silty, silty-clay.
This intervention refers to plant material and crop remains which have been brought in from elsewhere, rather than being left as ‘residues’ in the soil or on the soil surface after harvest of the valuable crop part (see also ‘Retain crop remains’). Soil microbial biomass is the amount of tiny living organisms in a given amount of soil and is measured by the amount of carbon or nitrogen they release into the soil. Soil aggregates are groups of soil particles held together by moist clay, organic matter (such as roots), organic compounds (from bacteria and fungi) or fungal hyphae (long, branching structures of a fungus). Some soil particles fit closely together, some do not, creating different-sized spaces. These spaces (or pores) within and between soil aggregates can store air and water, microbes, nutrients and organic matter. Large aggregations of particles retain the most nutrients. Nitrogen immobilisation (or demineralisation) is the conversion of inorganic compounds (e.g. nitrogen) to organic compounds by micro-organisms (e.g. incorporating it into cells), making it unavailable to plants. In this intervention, immobilisation is being encouraged after cropping to reduce nitrogen leaching. Macroorganic matter is defined as the fraction of soil organic matter with particles larger than 150 μm, made up of fragmented plant residues and microbial debris. Although a small component of soils (macroorganic matter makes up 3-6% of total soil organic matter), it is often called the ‘active’ fraction as it is linked to water availability and nutrient supply for both plants and microorganisms.
Supporting evidence from individual studies
A randomized, replicated experiment in 1979-1988 on clay loam in Alberta, Canada (Singh et al. 1994) found higher soil organic carbon (5.81%) under no tillage plus straw mulch and with tillage plus straw incorporation (5.79%) compared to tillage with no straw treatment (5.5%). Differences between treatments became less pronounced with increased soil depth. Soil aggregates were 38% larger in no tillage plus straw than tillage plus straw treatments, and 175% larger than tillage with no straw. The wind-erodible fraction of soil aggregates (aggregates smaller than 1 mm diameter) was smallest (16%) in no tillage plus straw (meaning soil structural stability was higher), followed by tillage plus straw (29%) compared to tillage with no straw (49%). The effects of tillage and straw remains were not separated. Three tillage and straw treatments were applied to a spring barley Hordeum vulgare crop. Treatments included: no tillage (direct seeding) and straw retained on the soil surface; tillage (rotavation to 10 cm depth in autumn and spring) and straw incorporated into topsoil; and tillage with straw removed. Individual plots measured 6.8 x 2.7 m and were replicated four times. Nitrogen was applied at 56 kg N/ha in all treatments. Soils were sampled to 5 cm depth.
A controlled, randomized, replicated experiment in 1991-1992 on fine loam in California, USA (Wyland et al. 1995) found higher microbial biomass and available nitrogen in soil with rye Secale cereal incorporated (23 and 35 μg N/g dry soil), compared to bare soil (10 and 20 μg N/g respectively), after three passes with a disc plough. There were two winter treatments: a rye Secale cereale crop sown in December then incorporated into the soil after16 weeks using a disc plough, and bare-fallow. Both treatments received the same tillage treatment (regular passes with a disc plough). Plots were 8 x 4 m. There were three replicates. Soil was sampled prior to rye incorporation (60 cm depth).
An experiment in 1994-1995 on silty clay in the Philippines (Witt et al. 2000) found 13-20% higher grain yields under early compared to late crop remains incorporation, without nitrogen or with low rates applied. There were two crop systems: continuous rice Oryza sativa, and a maize Zea mays-rice rotation. Maize was grown in the dry season, and rice in the dry. Within each 12 x 25 m cropping system were four 12 x 8 m nitrogen treatments: control (no nitrogen fertilizer), low (30 kg N/ha), medium (40 kg N/ha) and high application (50 kg N/ha). Within these were two 6 x 8 m sub-treatments: early (63 days before rice seedling transplanting) or late crop remains (rice or maize) incorporation (14 days before transplanting). Soils were sampled to 15 cm depth.
A randomized, replicated experiment in 1996-1998 on a sandy, silty and clay soil in Ludhiana, India (Jalota et al. 2001) found that straw incorporation was better in rain-free conditions (26.7 cm) and rainy conditions (22.2 cm) in medium coarse-textured soils compared to untreated soil (24.2 and 21.1 cm). In the coarsest soil, tillage and straw mulching did not increase soil water storage any more than untreated soil. Below the tillage and straw incorporation treatments, soil water content was higher (0.1318 and 0.1314 m3 water/m-3 soil, respectively) relative to the untreated and mulched soils (0.1059 and 0.1180 m3/m3). There were four treatments on three soil types: untreated, tilled to 8 cm depth, straw mulch (rice Oryza sativa in September and wheat Triticum aestivum in April) at 6 t/ha, and straw incorporation. The treatments were replicated three times in 2.5 x 3.5 m, 5 x 3 m and 6 x 4 m plots, for fine, medium coarse, and coarse soil respectively. Mechanical weeding or herbicides (glyphosate) kept plots weed free. Soil water content was measured every 15-20 days.
A controlled, randomized, replicated experiment from 1984 to 1997 on loamy sand and sandy loam in the UK (Silgram and Chambers, 2002) found higher soil mineral nitrogen under burned incorporated straw (51 kg N/ha), then chopped incorporated straw (46 kg N/ha) compared to no incorporation (no straw incorporation was not reported). Overall nitrogen increased under straw incorporation (633 and 429 kg N/ha at Gleadthorpe and Morley respectively). In wet winters, straw incorporation reduced nitrate leaching by 25 kg N/ha/y compared to not incorporating straw. Chopped straw reduced nitrate leaching by 12 kg N/ha/y compared to burned straw. There was no difference in grain yield between straw treatments. There were three residue treatments at two sites: burned straw incorporated (to 15 cm depth), chopped straw incorporated (15 cm depth), and chopped straw not incorporated. All treatments were mouldboard ploughed in autumn to 30 cm depth. Crops grown included: wheat Triticum aestivum, barley Hordeum vulgare, oats Avena sativa, sugar beet Beta vulgaris, winter oilseed rape Brassica napus. Grain and straw samples were used to measure nitrogen content. Soils were sampled to 90 cm depth.
A replicated experiment from 1981 to 2002 on sandy loam in Denmark (Thomsen and Christensen, 2004) found 30% higher soil carbon under high, 21% more under medium and 12% more under low straw application, compared to straw removal. Soil retained 14% of the carbon and 37% of the nitrogen from straw application. Higher grain yield was found under medium and high straw application (increased by 0.2-0.7 t/ha), compared to low or no straw application. Four straw management treatments were applied to barley Hordeum vulgare crops: straw removed (control: 0 t/ha), low (4 t/ha), medium (8 t/ha) and high straw application (12 t/ha). From 1981 to 1988, 35 t/ha of pig slurry was applied to the straw treatments. After, a ryegrass Lolium perenne catch crop was grown. Plot sizes were not specified for treatments above. In 1999-2002, wheat Triticum aestivum was sown into the straw treatments. Each treatment was divided into 21.25 m2 plots and received 0, 60, 120 or 180 kg N/ha. There were three replicates. Soil was sampled to 20 cm depth.
A controlled before-and-after, site comparison study from 1988 to 2002 on four sandy loam soils in Denmark (Kristiansen et al. 2005) found annual increases in soil carbon of 53-94 g/C/m2/year when maize Zea mays crop remains were incorporated into the soil, compared to 36-47 g/C/m2/year increases in three of four soil types receiving no amendment. Soil was collected from four arable fields (each with a different soil type) and placed outside in large, open-ended cylinders (0.7 m diameter x 0.5 m depth; number of cylinders not specified). After maize harvest, one cylinder for each soil type had chopped maize incorporated into the top 25 cm of soil. Remaining cylinders received no maize residue. Soil samples were taken every two-to-three years in spring to determine the amount of soil organic carbon.
An experiment in 1996 on sandy loam in India (Singh et al. 2005) found higher soil organic carbon under straw incorporation (0.53% of soil sampled) and burning (0.46%), compared to straw removal (0.42%). Soil porosity and soil aggregate size were also higher under straw incorporation (0.4 pores/sample, 0.34 mm) and burning (0.38 pores, 0.28 mm) compared to straw removal (0.37 pores and 0.24 mm respectively). Soil density was lower under straw incorporation (1.6 tm3) compared to straw burning (1.65 tm3) and removal (1.67 tm3). Water infiltration was higher under straw incorporation (0.57 cm/h) compared to straw burning (0.41 cm/h) or removal (0.53 cm/h). Three treatments were applied to a rice Oryza sativa-wheat Triticum aestivum rotation prior to wheat sowing: straw incorporated, straw burned, straw removed. Plot size was not specified. Soils were sampled to 15 cm depth, and soil cores were used to measure water retention.
A randomized, replicated experiment in 2003-2005 on silt loam, sandy loam and loamy sand in Flanders, Belgium (Chaves et al. 2007) found greater microbial biomass (measured by nitrogen quantities) under treatment with crop remains plus straw incorporation, with the largest increase in sandy loam (123 kg N/ha) then loamy sand (86 kg N/ha) and silt loam (98 kg N/ha), compared to a treatment using crop remains only (37, 34, 63 kg N/ha, respectively). Straw immobilized 96% of nitrogen released from crop remains in loamy sand, 76% in sandy loam, and 65-80% in silt loam. Crop remains plus green waste compost and sawdust did not affect microbial biomass or nitrogen immobilization. There were three replicates of 5.5 x 1.5 m treatment plots. Crop remains of cauliflower Brassica oleracea or leek Allium porrum were initially incorporated into plots along with a green waste compost on the loamy sand and silt loam. Sawdust was applied to the silt loam. After one year, crop remains plus cereal straw were incorporated into the soil instead, followed by barley malting sludge on the loamy sand and vinasses (residue from distillation of sugar) on the sandy loam. Soils were sampled regularly throughout the experiment to 90 cm depth.
A replicated experiment from 1990 to 2005 on silty-clay and sandy-silt in China and the Philippines (Bierke and Kaiser, 2008) found 41% higher soil organic carbon and total nitrogen at Changsha when residue was incorporated, compared to a 16% increase when residue was removed. There were no changes in soil organic carbon or total nitrogen at Nanjing or Los Baños. At Changsha there were three treatments: control (no fertilizer), nitrogen (applied at 265 kg N/ha), and nitrogen/phosphorus/potassium (NPK – applied at 265, 35, and 80 kg N, P, K/ha respectively). All plots were intercropped with Chinese milk vetch Astragalus sinicus. Within each treatment were two sub-treatments: residue removed, and residue incorporated (harvested rice Oryza sativa crop remains). At Nanjing, two treatments were applied to a rice-wheat Triticum aestivum rotation: residue removed and residue incorporated (wheat crop remains). All plots received 200 kg N/ha. Los Baños treatments included: continuous flooding, three week soil drying, soil drying plus tillage, and alternate wetting/drying of rice. Within these were ‘residue removed’ and ‘residue incorporated’ treatments. Soils were sampled between rice crops or just after wheat sowing. Plot sizes were not specified.
A before-and-after trial in 2003-2005 on loam/sandy loam soils in the UK (Ball & Crawford 2009) found that incorporating straw using reduced tillage (without ploughing) did not improve topsoil quality in compacted soils. Anaerobic growing conditions were found, shown by the high nitrous oxide flow to the air from below the straw layer (720 g N/ha/day) compared to above it (50 g N/ha/day). The carrot Daucus carota crop was part of a cereal/potato Solanum tuberosum/carrot/spring cereal rotation undersown with grass, clover Trifolium pratense and peas Pisum sativum. Carrot beds were roughly 2 m wide. Straw was incorporated into the soil before the carrot crop. Straw was incorporated using reduced tillage by discing to about 10 cm depth, without ploughing. Nitrous oxide and carbon dioxide fluxes were measured to determine whether soil conditions were anaerobic.
A controlled, replicated experiment in 2000 on sandy loam soil in Wellesbourne, UK (Rahn et al. 2009) found that adding sugar beet Beta vulgaris tops with molasses or compost to barley Hordeum vulgare increased soil mineral nitrogen by 46 and 11 kg/ha, and yield by 32% and 11% respectively, compared to no addition. Adding paper waste with sugar beet tops did not affect soil mineral nitrogen but improved yield by 23%. Adding sugar beet tops combined with straw, compactor waste or double rates of compactor waste reduced soil mineral nitrogen by 25, 15 and 36 kg/ha, and reduced yield by 47%, 21% and 63%, respectively. Amendments were applied at 3.2-3.8 t/ha, including compactor waste (produced by waste-compacting machines) and paper waste from the recycling industry, recently harvested wheat Triticum aestivum straw, compost from municipal green waste, and liquid molasses (thick brown, uncrystallized juice from raw sugar) from the sugar refining industry. Amendments were applied with 42 t/ha sugar beet tops.
A controlled, randomized, replicated experiment in 2001-2011 on sandy clay loam in India (Bhattacharyya et al. 2012) found 43.5% higher soil carbon under rice straw plus green manure (using Sesbania aculeata) compared to the control (5.16 g/kg). Microbial biomass (measured by quantities of carbon) was highest under farmyard manure plus green manure (250 mg/kg), followed by farmyard manure alone (233 mg/kg), compared to the control (153 mg/kg). Rice yield was highest under farmyard manure plus green manure (3.51 t/ha), followed by rice straw plus green manure (3.35 t/ha), farmyard manure alone (3.25 t/ha), and green manure alone (3.09 t/ha), compared to the control (1.93 t/ha). Treatments were applied to plots of paddy 20 days before plots were planted with transplanted rice Oryza sativa (Geetanjali variety) seedlings. Treatments included: no amendment (control), farmyard manure, green manure, farmyard manure plus green manure, and rice straw plus green manure (both incorporated into the soil 20 days before seedling transplantation). There were three replications. Soils were sampled at the beginning and end of the experiment to 60 cm depth.
A controlled, randomized, replicated experiment from 1990 to 2005 on silty, silty-clay and clay soil at four sites in China (Tang et al. 2012) found higher soil organic carbon levels under nitrogen/phosphorus/potassium (NPK) plus straw (9.1 g C/kg soil) compared to NPK alone ()8.4 g C/kg) or the control (7.7 g C/kg). Three treatments were applied to long-term wheat Triticum aestivum-maize Zea mays rotations at four sites: control (no fertilizer), NPK (at 165-362 kg N/ha, 25-41 kg P/ha, and 68-146 kg K/ha), and NPK plus straw (at 2.2-6 Mg/ha)). Weeds were removed manually and crops were irrigated when necessary. Soils were sampled to 20 cm depth at each site in autumn after maize harvest, and before the next fertilizer application.
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